The present invention generally relates to micromachining processes and devices formed thereby. More particularly, this invention relates to a process of forming a micromachined tube (microtube) suitable for a microfluidic device, including but not limited to Coriolis mass flow sensors, density sensors, specific gravity sensors, fuel cell concentration meters, chemical concentration sensors, temperature sensors, drug infusion devices, fluid delivery devices, gas delivery devices, gas sensors, bio sensors, medical sensors, and other devices capable of making use of a stationary or resonating microtube.
Processes for fabricating resonant mass flow and density sensors using silicon micromachining techniques are disclosed in commonly-assigned U.S. Pat. Nos. 6,477,901, 6,647,778, 7,351,603 and 7,381,628. As used herein, micromachining is a technique for forming very small elements by bulk etching a substrate (e.g., a silicon wafer), and/or by surface thin-film etching, the latter of which generally involves depositing a thin film (e.g., polysilicon or metal) on a sacrificial layer (e.g., oxide layer) on a substrate surface and then selectively removing portions of the sacrificial layer to free the deposited thin film. In the processes disclosed in U.S. Pat. Nos. 6,477,901, 6,647,778, 7,351,603 and 7,381,628, wafer bonding and etching techniques are used to produce a micromachined tube supported above a surface of a substrate. The tube can be vibrated at resonance, by which the flow rate, density, and/or other properties or parameters of a fluid flowing through the tube can be measured.
According to one embodiment of U.S. Pat. No. 6,477,901, a tube is formed using p-type doped layers and selective etching techniques. The doped layers form the walls of the tube, and therefore determine and limit the size of the tube walls as well as the cross-sectional dimensions of the tube. According to another embodiment of U.S. Pat. No. 6,477,901, a tube is formed with the use of a silicon-on-insulator (SOI) wafer. The buried oxide layer of the SOI wafer is used as an etch stop in a manner that determines and can limit the thickness of the tube. In U.S. Pat. No. 7,351,603, an epitaxial wafer is employed to avoid the higher cost of SOI wafers.
The micromachined tubes produced by the processes disclosed in U.S. Pat. Nos. 6,477,901, 6,647,778, 7,351,603 and 7,381,628 have roughly rectilinear cross-section passages as a result of using an anisotropic dry etching technique, such as reactive ion etching (RIE), dry etching, or deep reactive ion etching (DRIE), or a wet etching technique if the wafer is formed of a (110) oriented silicon. As known in the art, anisotropic etching processes produce a substantially one-directional etch, yielding the vertical walls of the passages shown within the microtubes of U.S. Pat. Nos. 6,477,901, 6,647,778, 7,351,603 and 7,381,628.